Zoom lens system for use in a microfilm projection apparatus

Abstract

A zoom lens system comprising, in order from an enlargement side, a first lens unit of a negative power, a second lens unit of a positive power and a third lens unit of a negative power. The first lens unit is fixed during a zooming operation from a longest focal length condition to a shortest focal length condition. The second and third lens units are movable from the enlargement side to a reduction side during the zooming operation, with a distance therebetween changing from a decrease to an increase.

What is claimed is: 
     
       1. A zoom lens system comprising, in order from an enlargement side: a first lens unit of a negative power;   a second lens unit of a positive power, and   a third lens unit of a negative power,   wherein, during a zooming operation from a longest focal length condition to a shortest focal length condition, said first lens unit is fixed, whereas said second lens unit and said third lens unit are movable from the enlargement side to a reduction side, with the distance between said second lens unit and said third lens unit first decreasing, then increasing, during said zooming operation, the zoom lens system satisfying the following conditions:   1.7<(Ψ.sub.I +Ψ.sub.II)/Ψ.sub.L <2.2       2.5<Ψ.sub.II IIIL /Ψ.sub.L <3.0        where Ψ I  represents a refracting power of said first lens unit, Ψ II  represents a refracting power of said second lens unit, Ψ L  represents a refracting power of the entire system in the longest focal length condition, and Ψ II  IIIL represents a combined refractive power of said second and third lens units in the longest focal length condition.   
     
     
       2. A zoom lens system comprising, in order from an enlargement side: a first lens unit of a negative power which includes a positive lens element at a position closest to the enlargement side;   a second lens unit of a positive power, and   a third lens unit of a negative power,   wherein, during a zooming operation from a longest focal length condition to a shortest focal length condition, said first lens unit is fixed, whereas said second lens unit and said third lens unit are movable from the enlargement side to a reduction side, with the distance between said second lens unit and said third lens unit first decreasing, then increasing, during said zooming operation.   
     
     
       3. A zoom lens system as claimed in claim 2, wherein said third lens unit includes a negative lens element at a position closest to the enlargement side. 
     
     
       4. A zoom lens system as claimed in claim 2, wherein said third lens unit includes a lens element having a convex surface facing the reduction side at a position closest to the reduction side. 
     
     
       5. A zoom lens system as claimed in claim 2, further comprising an aperture stop disposed adjacent said first lens unit. 
     
     
       6. A zoom lens system as claimed in claim 2, wherein said second lens unit includes, in order from the enlargement side, two positive lens elements, a lens element having a concave surface faced to the reduction side, and a positive lens element. 
     
     
       7. A zoom lens system comprising, in order from an enlargement side: a first lens unit of a negative power;   a second lens unit of a positive power, and   a third lens unit of a negative power,   wherein, during a zooming operation from a longest focal length condition to a shortest focal length condition, said first lens unit is fixed, whereas said second lens unit and said third lens unit are movable from the enlargement side to a reduction side, with a distance between said second lens unit and said third lens unit first decreasing, then increasing, during said zooming operation, and an aperture stop is disposed adjacent said first lens unit during the zooming operation.   
     
     
       8. A zoom lens system as claimed in claim 7, wherein said third lens unit includes a negative lens element at a position closest to the enlargement side. 
     
     
       9. A zoom lens system as claimed in claim 7, wherein said third lens unit includes a lens element having a convex surface facing the reduction side at a position closest to the reduction side. 
     
     
       10. A zoom lens system as claimed in claim 7, wherein said second lens unit includes, in order from the enlargement side, two positive lens elements, a lens element having a concave surface faced to the reduction side, and a positive lens element. 
     
     
       11. A zoom lens system comprising, in order from an enlargement side: a first lens unit of a negative power;   a second lens unit of a positive power including, from the enlargement side, two positive lens elements, and   a third lens unit of a negative power including a negative lens element at a position closest to the enlargement side,   wherein, during a zooming operation from a longest focal length condition to a shortest focal length condition, said first lens unit is fixed, whereas said second lens unit and said third lens unit are movable from the enlargement side to a reduction side, with the distance between said second lens unit and said third lens unit first decreasing, then increasing, during said zooming operation.   
     
     
       12. A zoom lens system as claimed in claim 11, further comprising an aperture stop disposed adjacent said first lens unit. 
     
     
       13. A zoom lens system comprising, in order from an enlargement side: a first lens unit of a negative power;   a second lens unit of a positive power including, from the enlargement side, two positive lens elements, and   a third lens unit of a negative power including a lens element having a convex surface facing the reduction side at a position closest to the reduction side,   wherein, during a zooming operation from a longest focal length condition to a shortest focal length condition, said first lens unit is fixed, whereas said second lens unit and said third lens unit are movable from the enlargement side to a reduction side, with the distance between said second lens unit and said third lens unit first decreasing, then increasing, during said zooming operation.   
     
     
       14. A zoom lens system as claimed in claim 13, further comprising an aperture stop disposed adjacent said first lens unit.